The invention is for an apparatus for transporting individual sheets past a device for exposing of or printing on the individual sheets.
In patent DE 196 36 235 A1, an apparatus for recording information on photographic material, which can be developed thermally, is described. In this apparatus, a heated roller is provided around which a sheet is deflected and which roller rotates each individual sheet by about 180xc2x0. In the first part of the deflection, the individual sheet is fixed to the roller by means of four pressure rollers. Between these pressure rollers there is an exposure gap through which a laser beam writes on the individual sheet to expose it. After the exposure, the pressure function is taken over by an endless belt, which is looped around the roller in this second region of the deflection. The endless belt has the task of holding the sheet in close contact with the roller so that the best possible transfer of heat can take place between the roller and the sheet.
Patent DE 30 00 887 A1 discloses an ink jet printer in which individual sheets are detached from a roller by guiding fingers engaging parallel annular grooves.
It is an object of the invention to provide an apparatus using a roller for transporting individual sheets past an exposure gap. The apparatus includes corresponding means for guiding the individual sheets around the roller in such a manner that a continuous, jerk-free movement of the individual sheets is assured even in the case of different formats such as sheet thickness and especially of different widths of the sheets and jamming or tilting of the sheets is prevented.
In a preferred embodiment, a plurality of endless parallel belts guide each individual sheet, irrespective of its size, without slippage in close contact with the roller. The roller has annular grooves into which guiding fingers extend to engage below the front edge of the individual sheet so that the sheet is lifted from the surface of the roller without a jerking motion and can be supplied to a subsequent transporting unit.
In the apparatus, the space in which an exposure unit is located is heated during a prolonged operation to a temperature, typically of about 35xc2x0 C. The individual sheets, which are of a light-sensitive material, are usually cut off from rolls in light-tight cassettes or the sheets are housed in light-tight cassettes when they are not being exposed. The cassettes and the light-sensitive materials in them are at normal room temperature, typically of about 20xc2x0 C.
Due to the contact with the roller, which can become heated by heat from the exposure device or other sources, the paper heats up. It was found that individual sheets are heated up far less rapidly at places at which they do not come into contact with the roller. In the present invention, this is accomplished by providing annular grooves in the roller at places over which the sheets run and, as a result, also are sensitized less. This would normally express itself by dark stripes in the finished image.
The effect of the dark stripes is avoided in the present invention by positioning the annular grooves opposite endless belts which also become heated. In this way, each place of the paper is in contact at least with an endless belt or with the surface of the roller so that the individual sheets are heated at least from one side. As a result, temperature differences over the area of the light-sensitive layer of the sheet are minimized and the stripes in the image are avoided.
The endless belts are constructed as flat belts so that the pressure on the sheet is distributed over a relatively large area. The flat belts are somewhat broader than the annular grooves so that the light-sensitive sheet lies smoothly on the surface of the roller and cannot be pressed into the annular grooves. It has turned out that the heating of the paper from the upper side by the belts and the underside by the roller has far less of an effect then the absence of heating from one side. The overlapping between the roller grooves and the endless belts does not have a harmful effect at the lateral edges of the annular grooves and is not visible in the finished image.
In order to be able to ensure good contact between the roller and the individual sheet, several endless belts are provided before as well as after the exposure station and loop the roller in each case at an angle larger than 40 degrees of the 360 degree surface of the roller.
It has also turned out to be advantageous to dimension the diameter of the roller so that it is not less than 150 mm. At a smaller diameter, the internal stresses, for example, of the photographic paper would exert a force on the belts so that a reliable contact between the paper and the roller could no longer be assured.
Ideally, the flat belts have a width of between 8 and 12 mm, whereas the annular grooves have a width of 3 to 5 mm. An overlapping between the roller surface on the one side and the endless belts on the other of 2 to 4 mm should be maintained on either side of the annular grooves. In this way, it is ensured that the flat belt is guided to overlie on the edges of the annular groove without running the risk that the paper will be pressed into an annular groove. On the other hand, the overlapping is so slight that there is no danger of a local temperature peak due to heating from both sides.